The present invention relates to hydraulic transmissions, more particularly, to hydraulic torque converters.
The torque converter constituting the present invention can be used with particular advantage in motor vehicle transmissions. When interposed in the power path aft of the engine, the torque converter automatically provides an infinitely-variable change of torque on its output shaft, which is connected to the transmission shaft, and safeguards the components of the engine and transmission from dynamic loads arising in going over rough terrain and in making gearchange, from engine torsional oscillations, and from other detrimental conditions. The torque converter is sufficiently simple to manufacture and reliable in operation.
The torque converters known in the art are constructed in the form of a vaned machine which usually comprises an impeller connected to the engine shaft, a turbine connected to the torque converter output shaft, and a stator designed to take up the reactive torque arising in the process of changing the engine torque and connected to a stationary shaft through a freewheeling clutch, the three abovesaid vaned members being arranged so as to form the working circuit of the torque converter.
Each of the vaned members consists of a casing, an inner ring, and vanes fitted therebetween.
All the vaned members are enclosed in the torque converter casing which is formed by the casing of one of the vaned members, usually the impeller, and associated parts. With this construction, the torque converter casing is rotatable.
When the impeller rotates, the working fluid contained in the torque converter is caused to flow through the vanes of the torque converter members and, by interaction therewith, rotates through 360.degree., circulating within the closed space. The flow of the working fluid is enclosed on the outside by the casings of the vaned members which jointly form the outer toroidal surface, whereas the inside enclosure is made up of the inner rings which jointly form the inner toroidal surface.
The most widely accepted type of torque converter employs a centrifugal impeller whose outlet is located at the maximum circulation radius, a centripetal turbine whose inlet is located at the maximum circulation radius, and a stator located at the minimum circulation radius.
The torque converters of this type are known to be widely used in the transmissions of automobiles and other vehicles. They have a compact construction and light weight and automatically provide change of torque over a wide range of speed ratios.
Concerning the torque converter vanes, it will be noted that the endeavour to cut down manufacturing costs, particularly in the automobile industry with its large-scale production, affects the hydrodynamic perfection of the vane profile. The deviation from the perfect vane profile adds to the power losses due to the vortex flow about the vanes. The resultant falling-off in the torque converter efficiency leads to decrease of the transmitted power and, consequently, adversely affects the performance of the vehicle involved.
Widely known in the prior art is a torque converter the casing of which accommodates a centrifugal impeller, a centripetal turbine and a stator, said three vaned members being aluminium alloy castings. The vaned members form a closed fluid circuit, their vanes being located between the vaned-member casing forming part of the outer toroidal surface and the inner ring forming part of the inner toroidal surface. The vanes, casing and inner ring are cast integrally. The vanes have varying sectional thickness and the vaned members feature high strength and operating reliability.
The torque converters of this construction are usually employed in the hydraulic transmissions of cargo trucks and other high-power vehicles.
However, the present-day sand-core technique used for casting torque converter vanes fails to provide the necessary accuracy of the vane profile and high finish of the surfaces in contact with the working fluid.
The typical drawbacks to cast vaned members are instability of the shape of the vane leading portion, excessive thickness of the trailing edge, roughness and other defects of the vane surfaces.
These drawbacks increase fluid frictional losses, vortex phenomenon and edge losses.
In consequence, the torque converter with cast vaned members has smaller efficiency and output/input ratio than the values that can be theoretically obtained with vanes having hydrodynamically perfect profiles.
The intricate form of the vane profile prevents the use of advanced precision casting methods such as high-pressure casting, etc. On the other hand, the correction of the casting imperfections by the file-bench technique increases the manufacturing cost. Therefore, the torque converter characteristics such as the efficiency and the torque ratio decrease with increasing production and, consequently, the performance of the vehicles involved becomes worse.
Also known in the art is a torque converter whose casing accommodates an impeller, a turbine and a stator arranged so that their vanes form a closed fluid circuit. The turbine vanes are located between the turbine casing which forms part of the outer toroidal surface and the inner ring which forms part of the inner toroidal surface. These vanes have varying sectional thickness and are connected to the impeller casing and inner ring by means of projections cast in the form of pins on the impeller casing and adapted to pass through a hole provided in each of the vanes and fit into slots provided in the inner ring. The fixing of the vanes is effected by plastic deformation of the pin ends projecting from the inner ring slots. Here, vanes with varying sectional thickness can be manufactured by the use of advanced casting technique which provides the necessary accuracy of the vane shape and high quality of the vane surface.
However, this construction suffers from the disadvantage that, in order to ensure the required strength and rigidity of the fixing pins, the casing of the vaned member has to be made of a high-strength material, for example, steel. This entails increase in the weight and in the moment of inertia of the vaned member, with consequent worsening of the vehicle accelerating properties.
Furthermore, the vanes have through holes for the fitment of the fixing pins and, therefore, their thickness is materially larger than in the case of the hydrodynamically perfect profile. Consequently, the flow passages between the vanes are substantially narrowed with resultant loss of power and decrease in the torque converter efficiency.
Also known and widely accepted is a torque converter whose casing accommodates a centrifugal impeller, a centripetal turbine and a stator arranged in such a manner that their vanes form a closed fluid circuit. The vanes of at least one of said vaned members are located between its casing which forms the outer toroidal surface and the inner ring which forms the inner toroidal surface. The casing and the inner ring having through slots. The vanes are attached to the casing and inner ring by means of tongues fitting through the slots. With this construction, the vanes, casing and inner ring are made by the use of the highly productive technique of sheet stamping. Constant-thickness vanes integal with tongues are cut out from thin sheet steel and shaped as necessary.
The casing and the inner ring are made in the same way, the slots being cut during the stamping process.
During assembly the vane tongues are fitted into the slots in the casing and inner ring and bent down, whereby a rigid structure is formed.
Most frequently the fixing of vanes by bending tongues fitted through slots in the casing and inner ring is employed on turbines. On impellers this method of vane fixing is used to a smaller extent inasmuch as the casing of an impeller usually serves as a torque casing and has, therefore, to provide a leaktight enclosure.
Since the vaned members of this construction are simple to manufacture and assemble and feature compactness, light weight, small moment of inertia and high operating reliability, they have found wide use in torque converters employed in car applications.
The high surface finish and the small thickness of the trailing edges of the vanes made by sheet stamping provide a sufficiently high torque converter efficiency under rated conditions in which the angles of the entering flow differ insignificantly from the vane angles.
When the fluid flow enters the vane at an angle different from the vane angle, a thin profile of constant thickness is conducive to greater losses than a hydrodynamic profile of varying thickness. This leads to decrease in the torque converter efficiency at off-design conditions and also in decrease of the torque conversion ratio, particularly at small speed ratios which are equal to the ratio of the turbine speed to the impeller speed. As a result, the torque conversion properties are impaired, resulting in decreased output power and, consequently, decreased vehicle speed.